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1968 firebird , highway RPM way to high

4K views 27 replies 8 participants last post by  starin 
#1 ·
Hi Guys and Gals,

So i have a 1968 firebird with a BOP TH350 it, i'm planning on throwing a 200r4 in it in a few months because the RPM on the freeway is so high. The th350 transmission is used, no idea how many miles. The rear diff is a gm 12bolt it was rebuilt before i bought it but no idea what the gears it has in it.

so at 56mph i am taching 3500 rpm and i have 28 inch tall tires, i used several gear calculators and everything says that equates to 5.21 gears. is it possible that my gearing in the diff can really be that high or maybe my transmission is not shifting into second gear??

thanks
 
#5 ·
X2 on everybody's advice, I had 4.88 in my 67 Firebird and at 60 MPH the RPM's where around 3,500. Top speed a wopping 107 MPH and that's what it hit going throw the traps at the end of the quarter mile.....once, I will not speak about the other times....LOL.

Count the teeth in your ring and pinion...do the math that way, then you know for sure. If your not sure about the transmission, try shifting it manually...see if your RPM's drop from 1st, to 2nd, to drive. If the RPM's don't drop from 2nd to drive, you don't have a 5.21 rear dif.

Ray
 
#6 ·
I don't know of any 5-plus ratio ring gear/pinions that are made for a 3-series carrier (4.88 is about the steepest for a 3-series) so you might look up the casting number to see if the housing is a 4- or 3-series.

Still best to count the teeth- that way you can see if it's a posi, the condition of the teeth can be seen, etc.

A word of caution: Be sure to use a W I D E pan to catch the diff lube! It stinks to high hell, and if you get it on the floor of the shop or driveway (let alone on you or your clothes), you'll regret it.:evil:
 
#7 ·
I might be wrong but in a 12 bolt diff, I think you could get into the 5's and even 6's...The 10 bolt was the one that the 4.88 was the steepest...like I said I could be wrong but, something just jogged my memory.

Oh, and if you spill gear oil on your clothes and your married and you bring it home to your wife to wash...in most states that's grounds for divorce.

Ray
 
#10 ·
A 12 bolt housing is a 12 bolt housing. The 2,3 and 4 series comes from the location of the ring gear flange on the differential. A 2 series differential has the flange offset to the left more than a 3 series and a 3 series is offset to the left more than a 4 series. This is to make up for the difference in the diameter of the pinion gear head for 2.56 to 6.xx gears. The car 12 bolt uses a 2 series differential for 2.56 and 2.73 gears, a 3 series for 3.07 to 3.73 gears and a 4 series differential for 3.90 to 6.xx gears. There are special thick 3.90, 4.10, 4.56 and 4.88 gears that are made to fit the 3 series and there is a special thin 3.73 that is made to fit the 4 series.
 
#12 ·
thanks for the help guys, when i first got the car last year it was a shell and i dropped the motor and trans in there when i was still living with my folks at there house. now i rent a town house and i'm not really allowed to work on it. i'll try the technique of spinning the tire and seeing how many times the driveshaft spins in one rotation.

i feel the trans shift but im not sure how many times, i know it shifts once but i can't feel the second time. the rpm on the highways definitely sucks
 
#19 ·
something just occured to me, the car never had a shifter so i installed the most basic shifter i could find which was a basic b&m. now i'm thinking is it possible i've been driving in d2 instead of D the whole time? how can i tell if im Drive and not in D2 the shifter is not the greatest.

thanks again for your guys input
 
#20 ·
You do have a tach, correct...what you could do, again, put the car in drive, watch your tachometer, you should see your RPM rise and drop twice...if the RPM's rise and drop from 1st to 2nd and then from 2nd to Drive...then you have all three gears. If it only does it once, then you are probably running in 1st and 2nd gear only.

Another way to tell is by listening to the engine, the same thing should happen, you will hear the engine rev up and then hear it drop and rev up again and drop...On a three speed automatic this should happen twice. If it's only happening once, you'll need to get under the car and adjust the shifter linkage.

Ray
 
#21 ·
Just an update because i always hate leaving hanging threads.

So i marked my driveshaft and tire with a white marker and spun the tire one rotation, the driveshaft spun 3.75 times, so i'm assuming i have 3.73 gears in it.


I then messed with my B&M automatic shift cable, the instructions say to put the trans linkage and shifter all the way in low 1. then put the car into park and make sure it's still in adjustment. Well when i had adjusted that the line on my shifter that indicates if the car is on D or R or N never reall lined up. so this time i manually moved the trans linkage to P then R and N and D etc. to make sure that each notch lined up with my shifter mark.

I havn't test drove the vehicle, can't until Saturday, but my guess hopefully is i was driving in D2 instead of Drive.
 
#24 ·
Ok so just an update and recap, i jacked up the car and spun the tire, for one rotation of the tire the driveshaft spun 3.75 times, so i guess that equates to 3.73 gears.

I also checked my shifter linkage and drive is drive and everything is good there.

I took the car for a drive and still the same issue at, 3500 rpm im doing 58 mph using my phone gps and and a tom tom GPS with 28 inch tall tires which equates to 5.03 gears. so i think i narrowed it down to some type of problem with the transmission.

Also when i was driving i did see that the rpm did drop twice once from 1st to 2nd and another from 3rd to 4th. but it shifted gears very quickly, it shifted from 1st to 2nd at 2000 rpm and then again from 2nd to 3rd around 2000 rpm. and both gear changed happened within 6 seconds of each other.

So i'm not too sure why my rear end gear 3.73 but its still taching high.

One other thing. i do not have the detent cable from the trans hooked up. when i dropped the pan intially to change the fluid the first time in this used th350 it had a little spring just hanging from the detent hook that connect to the detent cable. also when the detent was hooked up it would not retract. lets say if the carb was wide open throttle and the cable was pulled out, it would not retract when i let off the throttle and the car would just stay in first gear wind all the way up in rpm and then shift really hard. so i just pushed the cable back in. Don't know if this would have any relation as to why my trans is gicing me such high rpm.

Thanks again
 
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